At present, various medical devices are used. A PET device introduces a drug labeled with an isotope adapted to emit positrons into an organism and detects γ-rays derived from the drug with a plurality of detectors. The PET device includes a ring-shaped gantry (a frame), a cradle (a bed), and a computer for operation and the plurality of detectors arranged about the organism are embedded in the gantry.
An efficient detector for X-rays or γ-rays can be configured by combining a scintillator and an optical detector.
A CT/PET device obtained by combining an X-ray CT device and a PET device and a composite diagnostic device obtained by combining a magnetic resonance imaging (MRI) device with them have also been considered.
Optical detectors (photodiode arrays) employed in diagnostic devices such as those mentioned above are described in, for example, Patent Literature 1. A photodiode array such as a silicon photo multiplier (SiPM) or a pixelated photon detector (PPD) has a configuration in which avalanche photodiodes (APDs) are arranged in a matrix, a plurality of APDs are connected in parallel, and the sum of APD outputs is read. When operated in a Geiger mode, the APD can detect weak light.
That is, when photons are incident on the APD, a carrier generated within the APD is externally output through a quenching resistor and a signal read wiring pattern. A current flows through a pixel in which an electron avalanche is generated in the APD, but a voltage drop occurs in quenching resistors of about several hundred kΩ connected in series to the pixel. This voltage drop lowers the voltage applied to an amplification region of the APD, thereby terminating a multiplication action caused by the electron avalanche. Thus, the APD outputs one pulse signal when one photon is incident thereon.
In devices such as PET devices, a peak (an incident timing of a photon) of an output signal from an optical detector and a detected photon amount (sensitivity) are used and it is preferable for both the peak and the photo amount to have large values.